The formation of early-type galaxies

Abstract

This Thesis examines the formation and evolution of early-type elliptical and S0 galaxies. In order to shed light on the key processes which lead to the formation of elliptical and S0 galaxies a range of techniques have been employed. The kinematics and stellar populations of the major and minor axes of the nearby edge-on S0 galaxy NGC 3115 have been examined through deep Gemini-GMOS longslit spectroscopy. The behaviour of the radial profiles of the line strength indices is found to be well explained by a simple model where NGC 3115 is composed of two components; an older classical spheroidal component, which displays a negative (decreases outwards) metallicity gradient and is enhanced in α-elements relative to the solar value together with a younger, constant metallicity disc which displays close to solar [а/Ғе]. The kinematics and stellar populations of the integrated light of NGC 3115 are compared to those previously found for its globular duster (GC) system. The GCs are found to rotate in a manner consistent with the galaxy as a whole and the red GC sub-population is found to have ages, metallicities and [a/Fe] abundance indistinguishable from those displayed by the spheroid of NGC 3115 at 2 effective radii. This study provides strong support to the theory that the GC systems of galaxies are closely linked to galaxy spheroid formation. The formation and evolution of so galaxies is further examined through the study of 18 edge-on S0 galaxies observed in a manner identical to that presented for NGC 3115. These galaxies are all found to display significant stellar discs and to present regular disc kinematics. In several cases signatures of kinematic substructure such as inner discs or bars are found. Ionised gas emission is observed at a low level in 72% of the sample; the kinematics of this ionised gas is highly variable, with examples of regular disc-like kinematics, confused counter-rotation and streams not aligned with any axis observed. The kinematic data has been utilised to produce a B-band Tully-Fisher relation (TFR) for S0 galaxies. The S0 TFR is observed to be offset to fainter magnitudes from the TFR for local spirals, an observation which can crudely be explained by the formation of S0s through the truncation of star formation (and subsequent disc fading) in normal spiral galaxies. The offset from the spiral TFR for each galaxy is shown to correlate well with both disc and central age, in the correct sense, and with the correct magnitude, such that the large observed scatter in the S0 TFR can be explained as being due to the different times at which the progenitor spirals ceased forming stars and hence the different amounts they have faded up to the present. Multi-object spectroscopic observations of the GC system of the shell elliptical NGC3923 are presented, as well as a novel technique for examining the spectra of the integrated light of galaxies at the same time as observing their GC system. The observations are used to demonstrate that neither the integrated light nor the GC system of NGC 3923 shows evidence for significant rotation. The stellar populations of the red GCs are again found to be consistent in age, metallicity and [a/Fe] with that displayed by the integrated light of the spheroid of NGC 3923 at large radii (> 2R(_e)). The velocity dispersion profile of the integrated light and GC system of NGC 3923 are both observed to be flat at larger radii, a finding indicative of the presence of significant amounts of Dark Matter. Using the GC kinematic data, a simple spherical isotropic dynamical model is presented which demonstrates that a significant increase in mass-to-light (M/L) ratio is required to explain the observed constant velocity dispersion profile. The M/L profile determined is in good agreement with that measured independently from X-ray observations.